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Old 04-14-2007, 01:09 PM   #1
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Default Compression vs. Octane

I have been looking for some sort of chart or table that compares the static compression ratios of NA gasoline engines to required minimum fuel octane rating. Now obviously there is no one perfect octane rating for a specific compression ratio due to several variables, but there is a general range. This is not for my car, but for my quad, which I will be running 13.5-14:1 compression ratio on. I figure you guys would better know where to find this information. Thanks!
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Old 04-14-2007, 03:33 PM   #2
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my waverunner runs 11.4:1 compression and it runs the same on 87 as it does on 93.. if it helps?
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Old 04-14-2007, 06:38 PM   #3
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Lots of factors play into it like aluminum or iron heads, quench, cooling capacity and so forth. The better question might be dynamic compression versus octane.
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Old 04-15-2007, 01:34 PM   #4
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another big factor not mentioned is how much timing you want, your not guuna be able to curve out 30 degrees on 87 octane
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Old 04-15-2007, 04:49 PM   #5
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As I said in the original post, I know there are several variables, but there is a general range, and I am looking for these ranges per given SCR.
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Old 05-13-2007, 07:06 PM   #6
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VP has something like that on the description of each of their fuels.

http://www.vpracingfuels.com/vp_01_fuels.html#leaded
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Old 05-14-2007, 01:30 AM   #7
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Quote:
Originally Posted by Ferocity02
As I said in the original post, I know there are several variables, but there is a general range, and I am looking for these ranges per given SCR.
the thing that really determines or has a major influence on octatine is dynamic compression ratio

think of it you can have two different engines with different SCRs but can have the same DCR meaning both have the same octane requirement.

your best bet on what octane requirement is look into DCR instead of SCR
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Old 05-15-2007, 03:55 PM   #8
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static compression ratio is only part of the equation. You can use it as a minimum limit, such that if you have a 8:1 or 8.5:1 static compression ratio (simple volume of cylinder at top dead center vs bottom dead center) then unless the motor has some crazy efficiency and has high cylinder pressures at top rpm, you will most likely not need high octane gas. General rule of thumb I see is less than 9:1 compression ratio always run 87 octane gas.
On the other end of the spectrum, you can run around 11:1 compression which is usually associates itself with high octane gas (91/93 and higher). But... what kind of cam your running has a big impact. How long the intake stays open on the compression stroke relieves compression, and how much valve overlap also plays into things, which is the dynamic compression ratio and what really determines required fuel octane. And which is also why motors can run 10:1 to 11:1 CR and get by on 87 octane gas, or why some can and others must use 93 octane or better.

it's actually cylinder pressure you want to know for octane requirement. I thought i remember hearing or reading that for regular pump gasoline, and not sure if that means 87 octane or up to 93 octane is pump gas, that cylinder pressures should be < ~ 200psi. I think that was for a static measurement, as with a CR gauge cranking the motor with the starter. What cylinder pressures are at rpm I don't know. Run a CR gauge off a cylinder and run the motor at 3000-4000 rpm and find out for us
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Old 05-15-2007, 03:55 PM
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